The present invention relates to polytetrafluoroethylene (hereinafter referred to as PTFE in some cases) fine powder having superior powder flowability. This PTFE fine powder is suitably used as a molding material for paste extrusion molding and the like, alternatively, as a resin additive for reforming the properties of a variety of resins.
Conventional PTFE fine powders have the properties of being apt to coagulate and thus exhibit poor working efficiency when they are taken out of containers for being used in molding or the like. Poor working efficiency can be seen, for example, when PTFE fine powders are mixed with an extrusion aid in the step of extruding rod or tube-shaped premolded articles by paste extrusion method. In addition, there are the following problems. In cases where PTFE fine powders are added to plastic for imparting sliding properties or antidripping properties, it is necessary to premix the PTFE fine powder with a material resin. At that time, however, the powders coagulate in a Henschel mixer or an extruder""s feeder to cause blocking, which considerably degrades working efficiency and productivity. in addition, since PTFE fine powders have poor dispersibility to material resins, silver lines caused by the segregated PTFE may occur on the surfaces of molded articles, depending: on the conditions of compounds. The term xe2x80x9csilver linexe2x80x9d is a silvery bar trace that generates on the surfaces of molded articles in the flow direction of resins.
Detailed description is given of the cases where PTFE fine powder are used as an antidripping agent.
As to the flame resistance regulations for plastics, the standards of various countries have been integrated in recent years. In Japan the regulations have been severer because of the tendency of product security. As the safety standards of electronic and electrical equipment, the regulations of UL (Underwriters""s Laboratories) in the USA have the outstanding contents, which is not the domestic standards in Japan, but have a significant influence. In these circumstances, the demand for flame resistive resins is expected to increase for the future.
Since much of thermoplastic resins used in electrical appliances and office machinery are flammable, flame retardants are usually added for improving the incombustibility of resin molded articles. To meet the requirements for miniaturization and thin-wall of molded articles, the regulations of UL 94 required for flame resistive resins have become severer in these years. In the latest notebook PCs, for example, the wall thickness does not exceed 1 mm so as to obtain 300-g frames for compactness and lightness. Conventional flame resistive resins can make molded articles non-flammable, however, if used for such thin articles, it is liable that once they begin to burn, thermoplastic resins are in the liquid state and drip while burning, and then spread. On the other hand, it is known to prevent dripping during burning by adding and kneading PTFE fine powder into thermoplastic resins. This utilizes the characteristics that PTFE fine powders are easily fibrilized.
As to flame retardants, bromine-containing flame retardant DBDE (decabromodiphenyl ether), has often used for high impact polystyrene (HIPS), ABS, polycarbonate (PC), polybutyleneterephthalate (PBT), etc. However, replacement of DBDE has proceeded rapidly because of dioxin that is considered to generate in burning DBDE. As a substitute, brominated epoxy oligomer is presently the most effective from the viewpoints of flame resistance and weather resistance. However, the bromine content of brominated epoxy oligomer is about two- thirds of that of DBDE, and thus it is required to increase the amount of addition. Furthermore, the price of brominated epoxy oligomer is about 1.5 times that of DBDE, thereby increasing the cost of flame resistive resins. Therefore, a reduction of flame retardant is a big subject. In this regard, it is known that flame retardant can be reduced if an antidripping function is imparted by the addition of PTFE fine powder. However, as described earlier, PTFE fine powders are generally apt to coagulate. Hence, the powders mixed with a material resin may solidify in a Henschel mixer or an extruder""s feeder during premixing, to cause blocking, which may often significantly deteriorate handling characteristics and productivity. Also, due two poor dispersibility, silver lines caused by the segregated PTFE may occur on the surfaces of molded articles, depending on the conditions of compounds.
Meanwhile, it is well known that fibrillating polytetrafluoroethylene is incorporated into powder having dusting characteristics to prevent dusting characteristics (JP-B-32877/1977, JP-B-24872/1993, JP-A-91993/1989 and JP-A-81882/1989).
In industrial manufacturing, economy, productivity and working efficiency must be taken into consideration. In order to uniformly disperse polytetrafluoroethylene in a desired concentration, it is desirable that polytetrafluoroethylene is supplied individually by an automatic weight or volumetric counting feeder.
However, due to poor powder flowability of conventional polytetrafluoroethylene, the jamming of powders has occurred in automatic weight or volumetric counting feeders, so that working efficiency is significantly reduced.
Accordingly, it is an object of the present invention to provide PTFE fine powders excellent in powder flowability which are used as molding materials, resin additive, or the like.
It is another object of the present invention to provide antidripping agents, excellent in handling characteristics and dispersibility while maintaining antidripping property, as well as flame resistive resin compositions containing the antidripping agent.
It is still another object of the present invention to provide compositions for inhibiting dusting characteristics that are excellent in handling characteristics and working efficiency.
The present invention relates to a polytetrafluoroethylene fine powder excellent in powder flowability which comprises fine particles (the primary particle) having a mean particle diameter of 0.05 to 1 xcexcm, an apparent density of 0.52 to 0.70 g/ml, a standard specific gravity (SSG) of 2.14 to 2.23, and a secondary mean particle diameter of 100 to 1000 xcexcm. The invention also relates to antidripping agents using the above polytetrafluoroethylene fine powder, flame resistive resin compositions containing this powder, and compositions, for inhibiting dust characteristics containing this powder.
The polytetrafluoroethylene fine powder of the invention is characterized in having specific mean particle diameters of the primary and secondary particles, specific apparent density and standard specific gravity, as defined above.
PTFE fine powder of the invention can be prepared by a method in which a polymer latex is obtained by known emulsion polymerization (JP-B-4643/1962, JP-B-14466/1971, JP-B-26242/1981, and U.S. Pat. No. 2,965,595), and a surfactant is then added into the latex when subjected to a coagulation.
It is not fully why the apparent density of PTFE fine powder increases by the addition of surfactant in coagulating the latex after the reaction, however, it is estimated that the primary particles within the secondary particles are closely filled when water evaporates in the step of drying. As a result, powders in which particles are highly packed with a high apparent density are obtained, improving powder flowability.
The fibrillating properties of PTFE fine powder are the characteristics that can be normally recognized in the PTFE whose molecular weight is too high to perform melt molding (i.e., the melt viscosity at 380xc2x0 C. is more than 108 poise). Specifically, such PTFE have a standard specific gravity (ASTM D-1457) of not more than 2.23, preferably in the range of 2.14 to 2.23 (as the value of standard specific gravity decreases, the molecular weight increases). Over 2.23, i.e., in lower molecular weights, it is less susceptible to fibrillation. On the contrary, the PTFE having a high molecular weight whose standard specific gravity is smaller than 2.14 do not lose the fibrillating properties inherent in high molecular weight PTFEs. However, their manufacturing is difficult and thus not practical.
In the polytetrafluoroethylene fine powders of the invention, even if fine powder comprising fine particles of a modified PTFE is used, such a modified PTFE is included in the PTFE fine powder of the invention, without deteriorating fibrillating properties (JP-B-26242/1981, JP-B-3765/1992 and JP-A-1711/1989).
Here, the term xe2x80x9cmodificationxe2x80x9d means that other fluorine-containing monomer is copolymerized in a small amount when polymerizing tetrafluoroethylene. The modified amount is so adjusted that the copolymerized monomer content is about 0.001 to 2 wt %, preferably 0.01 to 1 wt % , more preferably 0.03 to 0.3 wt %, to the entire polymer fine particles to be generated.
As the fluorine-containing monomer for copolymerization used in modification, there are generally employed at least one of chlorotrifluoroethylene (CTFE) hexafluoropropene (HFP), perfluoro (alkyl vinyl ether) (PFAVE), and the like.
As surfactants used in the present invention, there are fluorine-containing surfactants, e.g., fluorine-containing anionic surfactants, fluorine-containing nonionic surfactants, fluorine-containing cationic surfactants, fluorine-containing betainic surfactants; and hydrocarbon-containing surfactants, e.g., hydrocarbon-containing nonionic surfactants, hydrocarbon-containing anionic surfactants.
Examples of fluorine-containing anionic surfactants include compounds of the formula (1). There are for example CF3(CF2)6COONH4, CF3(CF2)7COONa, and H(CF2CF2)4COONH4
wherein R1 is F or CF3, R2 is H, F or CF3, n is an integer of 4 to 20, m is 0 or an integer of 1 to 6, R3 is COOM or SO3M, M is H, NH4, Na, K or Li.
Examples of fluorine-containing nonionic surfactants include the compounds of the formula (2) 
wherein R1, R2, n and m are as defined above, k is 0 or 1, R4 is H, CH3 or OCOCH3, R5 is (OCH2CH2) pOR6, p is 0 or an integer of 1 to 50, R6 is H, C1xcx9cC20 alkyl or C6xcx9cC26 aromatic group.
Examples of fluorine-containing cationic surfactants include the compounds of the formula (2) wherein R5 is a group of the formula (3). 
Examples of fluorine-containing betainic surfactants include compounds of the formula (2) wherein R5 is a group of the formula (4). 
Examples of hydrocarbon-containing nonionic surfactants include polyoxyethylene alkyl ether, polyoxyethylene alkyl ester, polyoxyethylene alkylphenyl ether, sorbitan alkylate, and polyoxyethylene sorbitan monoalkylate.
Examples of hydrocarbon-containing anionic surfactants include alkylcarboxylate, alkylsulfonate, alkylsulfate, alkylbenzene sulfonate, alkylphosphonate, and alkylphosphate.
Among these surfactants, preferred are fluorine-containing surfactants, particularly fluorine-containing anionic surfactants, because they can contribute to the effect in small amounts. In the present invention the above-mentioned surfactant is added in the range of 0.01 to 10.0 wt % preferably 0.1 to 5 wt % , to the polymer in latex.
In the process of coagulating PTFE fine powder, latex obtained from polymerization is placed into, for example, a vessel equipped with an agitator and baffle boards, and a surfactant is then added in the range of 0.01 to 10.0 wt %, preferably 0.1 to 5 wt %, to the weight of the polymer in the latex. Thereafter, the latex is diluted with water so that the polymer concentration ranges 5 to 50 wt %, preferably 10 to 20 wt %, and if required, the latex pH is controlled so as be neutral or alkali. The temperature of the latex is adjusted to 0 to 80xc2x0 C. , and its specific gravity is adjusted to 1.03 to 1.20. The latex is then agitated more vigorously than that during the reaction. It is necessary to determine an appropriate revolutions for agitation, depending on the size of vessel, the shape of mixing blades, the structure of baffle boards, and the like. Here, the agitation may be carried out while adding, as a coagulant, water soluble organic compounds, e.g., methanol and acetone; inorganic salts, e.g., potassium nitrate and ammonium carbonate; or inorganic acids, e.g., hydrochloric acid, sulfuric acid and nitric acid. With agitation, the latex particles become unstable and then separate from the aqueous phase simultaneously with coagulation. Thereafter, washing is performed as required, to remove emulsifier, surfactant and coagulant, followed by drying.
Drying is performed by means of vacuum, high frequency, hot blast, or the like, while restraining the flow of the wet powder obtained from coagulation, preferably, while maintaining it stationary. Drying temperature ranges 10 to 250xc2x0 C., preferably 100 to 180xc2x0 C .The particle diameter of the powder can be controlled by the solid concentration and temperature of the latex during coagulation, and the frequency 10 of agitation. The powder is an aggregate comprising fine particles of 0.05 to 1 xcexcm. Its mean particle diameter is the secondary one, and suitable mean particle diameters range 100 xcexcm to 1000 xcexcm, more preferably 300 xcexcm to 700 xcexcm.
The polytetrafluoroethylene fine powder of the present invention can be employed as an antidripping agent for giving antidripping properties to flammable thermoplastic resins.
The present invention also relates to flame resistive resin compositions comprising 100 parts by weight of the above-mentioned flammable thermoplastic resin, 0.01 to 5 parts by weight of the above-mentioned antidripping agent, and 0.001 to 40 parts by weight of flame retardant. Examples of flammable thermoplastic resin include polyolefin resins (polyethylene, polypropylene, polymethylpentene, etc) polyvinyl chloride, polystyrene-containing resins (polystyrene, AS, ABS, etc), polycarbonate (PC)-containing resins (PC, PC-containing alloy resins, e.g., PC/ABS, etc), polyamide-containing resins (.nylon, whole aromatic polyamides, etc) polyester-containing resins (polybutyleneterephthalate, polyethyleneterephthalate, whole aromatic polyesters, etc), acrylic-containing resins [poly(methyl methacrylate), polyacrylonitrile, etc], polyacetal, polyether ether ketone, modified polyphenylene ether, polyarylene sulfide resin, polysulfone resin, and a variety of polymer alloys.
Outstanding antidripping effects are obtained particularly when incorporated into resins, such as PC, PC alloys, polystyrene, polybutyleneterephthalate, polyethyleneterephthalate, and modified polyphenylene ether, which are used in applications requiring a higher flame resistance, e.g., housing parts and various mechanism elements for domestic electric appliances and office machinery.
In the present invention the addition of antidripping agent ranges from 0.01 to 5 parts (part by weight, same as hereinafter), preferably from 0.03 to 2 parts, per 100 parts of a flammable thermoplastic resin. Less than 0.01 part, there is a trend that a desirable antidripping properties not to be obtained. More than 5 parts, antidripping properties, mold-releasing properties and friction properties of molded articles, are improved, however, it is liable to cause poor dispersion of the antidripping agent in the resin.
Representative flame retardants are, for example, compounds containing an atom of Group 5B of the Periodic Table, such as nitrogen, phosphorus, antimony and bismuth, and compounds containing a halide of an atom of Group 7B. Examples of halide include aliphatic halides, alicyclic halides, and aromatic organic halides. There are exemplified bromine-containing halides such as tetrabromobisphenol A (TBA) decabromodiphenyl ether (DBDPE), octabromodiphenyl ether (OBDPE), TBA epoxy/phenoxy oligomer and brominated crosslinked polystyrene; chlorine-containing halides such as chlorinated paraffin and perchlorocyclopentadecane. Examples of phosphorus compound include phosphate and polyphosphates. It is preferable that antimony compounds are jointly used with halide. Examples of antimony compounds include antimony trioxide and antimony pentaoxide. Besides these, aluminum hydroxide, magnesium hydroxide and molybdenum trioxide are also usable. The flame retardant is not limited to the above, and it is possible to suitably select at least one from these flame retardants and to determine the amount thereof, depending on the type of flammable thermoplastic resins. The amount of the above-mentioned flame retardants is usually from 0.001 to 40 parts, preferably from 0.01 to 30 parts, per 100 parts of the above-mentioned resin. Less than 0.001 part, the flame resistance tends to be insufficient. More than 40 parts, it is not economical and the mechanical characteristics (impact resistance, etc) of the resin composition tends to deteriorate.
The flame resistive resin composition of the present invention comprises a flammable thermoplastic resin, the antidripping agent of the present invention, and a flame retardant. These compositions can be prepared by blending the ingredients by known methods, without limiting the conditions, e.g., the order of blending; as to whether blending is conducted in the powder or dispersion state; the type of blending machines and the combination, and the like. For example, after a flame retardant is blended with the antidripping agent of the invention, the mixture is fed into a kneader together with a flammable thermoplastic resin. Alternatively, the antidripping agent of the invention is blended with a flammable thermoplastic resin, part or all of which an aqueous dispersion or organosol. Without limiting these methods, a variety of blending methods can be employed.
To the flame resistive resin compositions of the invention, there can be added, in addition to a flame retardant and the antidripping agent, known additives, such as ultraviolet absorber, antioxidant, pigment, molding aid, calcium carbonate and glass fiber, as required.
The present invention also relates to powder compositions whose dusting characteristics is suppressed, which contains 0.005 to 1.0 part by weight of the polytetrafluoroethylene fine powder of the invention per 100 parts of powder having dusting characteristics.
As the powder having dusting characteristics, there are, for example, Portland cement compositions used in a variety of concrete structures and also used as a soil improver, quick lime used as a soil improver, slaked lime, calcium silicate used as a fertilizer, calcium carbonate and like flyashes, slug, ferrite, asphaltite, dust collection in steel furnaces, silica gel, alumina, pigment, carbon black, talc, active carbon, flame retardant, antimony oxide, and powder and granular waste, e.g., red mud and sludge.
The polytetrafluoroethylene fine powder of the invention is blended in the range of 0.005 to 1.0 part, preferably 0.01 to 0.5 part, per 100 parts of powder having dusting characteristics. Below 0.005 part, there is a tendency for a desirable dust inhibiting effect not to be obtained. Over 1.0 part, the effect is saturated and thus unfavorable from economic considerations.
Accordingly, due to. superior powder flowability of the polytetrafluoroethylene fine powder of the invention, it is possible to solve the jamming of powders in automatic weight or volumetric counting feeders. This permits a uniform dispersion of the powder, enabling the desired effect to be produced with less addition than that of conventional polytetrafluoroethylene.
Also, due to superior powder flowability of the polytetrafluoroethylene fine powder of the invention, it is possible to mix the powder uniformly with electrode materials, for example. Furthermore, since this powder does not swell with an organic electrolyte and has superior binding properties, it is possible to use the powder as binders for batteries.